1. Field of the Invention
The present invention relates to a humidity sensor for measuring the moisture content of an atmosphere by means of change in the electrical resistance of a moisture sensitive layer. The humidity sensor can be used, for example, for measuring the moisture content of air; the moisture content of exhaust gas of an internal combustion engine of, for example, an automobile, ship, or airplane, particularly the moisture content of an atmosphere containing substantially no oxygen and containing a reducing gas; or the moisture content of a highly reducing atmosphere surrounding a fuel electrode or an air electrode of a fuel cell.
2. Description of the Related Art
Commercially available humidity sensors include resistance-variable-type humidity sensors utilizing adsorption and desorption of water molecules; and capacitance-variable-type humidity sensors utilizing a change in capacitance. Such humidity sensors are formed, for example, from a moisture sensitive ceramic material such as Al2O3, MgCr2O4—TiO2, TiO2—V2O5, or ZrCr2O4—LiZrVO4. Most of the humidity sensors measure moisture content by means of a change in electric resistance.
A typical humidity sensor formed from such a moisture sensitive ceramic material includes an insulating substrate, a lower electrode formed on the substrate, a moisture sensitive layer provided on the lower electrode, and an upper electrode formed on the moisture sensitive layer. In many cases, the lower and upper electrodes are formed from a porous body of RuO2 in order to enhance the strength of adhesion between the insulating substrate and the moisture sensitive layer, and to cause water vapor contained in air to easily pass through the moisture sensitive layer easily.
3. Problems Solved by the Invention
When an electrode formed from RuO2 is exposed to a reducing atmosphere in an exhaust pipe of an automobile or in a specific portion of a fuel cell where the oxygen content is very low and temperature changes over a wide range (e.g., −40° C. to 750° C.), the electrode may be impaired with the passage of time. The problem then arises that the humidity sensor cannot detect a change in the resistance of the moisture sensitive material even though the humidity changes.